Welding is a process of joining metal pieces to each other by fusion and involves no metal removal. There are approximately 40 different welding processes used today. These processes can be generally categorized as follows: arc welding; gas welding; resistance welding; brazing; solid-state welding; and "power beam" welding. While each category of welding process has specific benefits relating thereto, the present invention is primarily concerned with "power beam" welding techniques. As used herein, the term "power beam" welding is meant to include electron-beam welding, plasma-arc welding, laser beam welding and/or other "high intensity" welding techniques.
As the power beam impacts and penetrates into the metal pieces, kinetic or electromagnetic energy is converted into thermal energy thereby causing melting of interfacing surfaces on the metal pieces. When solidification of the molten metals occurs, a fusion zone or weld joint results.
Power beam welding processes offer unique performance capabilities and provide a solution to a wide range of joining problems. Advantageously, power beam welding requires no metal removal thereby reducing labor and costs of the joining process. Power beam welding techniques produce a weld joint with a cross-section that is deeper and narrower than those normally resulting from other welding techniques. The ability to attain an extremely high weld depth-to-width ratio permits single-path welding of heavy metal sections. The total heat input per unit length for a given depth of penetration can be much lower when using power beam welding techniques as compared to other welding techniques. This yields a much narrower heat-affected zone, noticeably less distortion, and fewer thermal effects compared to other welding techniques. When conducted in a vacuum or high purity environment, power beam welding minimizes contamination of the metal by oxygen and nitrogen. Moreover, the ability to project a power beam over an elongated distance allows welds to be made in otherwise inaccessible locations.
With power beam welding, rapid travel speeds are possible because of the high melting rates associated with the concentrated heat source. This reduces the time required to accomplish welding and increases the productivity and energy efficiency of the process. Moreover, reasonably square butt joints in thick metal pieces can be welded in one pass without the addition of filler metals. Because the density of the power beam produces welds that are not controlled by thermal conduction, metals of significantly different thermal conductivities can be welded together. As will be understood, other welding techniques or procedures often require the use of preheat for thick sections of metal having a high thermal conductivity, such as aluminum or copper.
Although power beam welding is a high power density process, it is also a low energy process. That is, the energy required by a power beam to form a weld of a given thickness is considerably less than that required by more conventional welding processes. Two advantages can result from the low energy input. First, it minimizes distortion and reduces the size of the weld heat-effected zone. Second, the high cooling rate associated with narrow power beam welds can effect metallurgical reactions, such as phase changes. Although the fundamental rules of metallurgy regarding cooling rates and the resulting microstructure still apply, power beam weld metal will have mechanical properties normally associated with the bulk properties of the microstructure.
The intensity of the power beam is controlled and is capable of instantly penetrating into the work piece and forming molten metal. Metal immediately adjacent to the molten metal is first heated; it expands against the restraining forces of the surrounding cold or boundary base metal; then it cools and contracts as it solidifies. In effect, metal is plasmically deformed during the heating cycle and restrained in tension during cooling. Accordingly, a series of residual tensile and compressive stresses surround the weld joint.
While the strength of weld joint formed with a power beam is usually higher than that of the parts to be welded, the strength of the finished weld joint can be adversely effected by cracking of the weld joint. As will be appreciated, a weld joint having cracks therein may fail when the parts are placed under load. When the welded parts are housed within a casing such as a transmission housing, the failure of the weld joint can result in costly and time involved repairs. Of course, the quality of a weld is difficult to determine without extensive testing procedures.
Thick sections of hardenable steels may crack when power beam welding techniques are used to form the weld joint. Very rapid cooling in the fusion and heat-effected zones also tend to cause shrinkage cracking of the weld joint.
Hot or cold cracks may occur in power beam weld joints in alloys that are subject to these types of cracking. Hot cracking is generally intergranular, and cold cracking is transgranular. Hot cracks form in a low-melting grain boundary phase during solidification of the weld metal. Cold cracks form after solidification as the result of high internal stresses produced by thermal contraction of the metal during cooling. A crack originates at some imperfection or point of stress concentration in the metal and propagates through the grains by cleavage. Stresses within the weld joint can lead to microcracks during weld solidification due to design restrictions on the parts to be welded. A circular weld joint may experience severe constraints.
Sulphur and phosphorous are materials used in many steels to facilitate machining. When the sulphur and phosphorous levels in the metal parts exceed certain levels, they promote low weld ductility and are susceptible to high contraction stresses. As will be understood, during a welding process, sulphur segregates to the grain boundaries and tends to cause cracking of the weld joint. Accordingly, one attempt at solving the weld cracking problem involves reducing the sulphur and phosphorous contents of the metal pieces being welded. Of course, reductions in sulphur and phosphorous levels adversely effect machining operations.
Moreover, when the weld joint is formed between abutting surfaces of two metal pieces, the weld joint often suffers from weld centerline cracking upon solidification. The cracking propagates quickly from the weld root and is substantially undetectable from an outer surface of the weld.
Relatively heavy sections of high strength alloy steels ordinarily must be preheated prior to power beam welding to prevent cracking. As will be appreciated, preheating involves a time consuming laborious process which adds to the overall cost of the welded assembly resulting from the welding process.
It is not unusual to harden functional surfaces on the workpieces to increase their life expectancy. A carburizing heat treatment is one method of hardening the metal pieces or parts. Carbon absorbed from the carburizing heat treatment of the metal pieces, however, assists in crack development. Preheating could prevent the development of high hardness in the weld zone but, to be effective, would probably have to significantly exceed the 180.degree. C. tempering temperature of the carburized parts and cause an unacceptable hardness loss on the carburized surfaces.
Thus, there is a need and a desire to capitalize on the numerous benefits and advantages offered by power beam welding techniques while inhibiting cracking of a weld joint during the weld solidification process.